The instant invention relates to a process for making open cell flexible polyurethane foam that comprises selecting certain carboxylic acids in combination with a specific group of tertiary amines gelling catalysts having isocyanate reactive groups to produce foam with relatively low chemical emissions.
The production of open cell flexible polyurethane foam requires the use of a variety of additives and each one of them plays a key role in determining the final characteristics and physical properties of the product. Although these additives represent a small percentage in the overall formulation and emissions are expected to be relatively low, the increasing environmental demand for low volatile organic contents (VOC's) in finished products has placed additional requirements on additives for lower emissions while maintaining foam performance. Thus, increased scrutiny of emissions related to catalysts, surfactants and any other component present in polyurethane foam formulation have become more common and this tendency is now global. Polyurethane manufacturers need additives with no emissions that can perform as well as the conventional less environmentally friendly standard products. It is well known that the conventional manufacture procedure to make polyurethane foams requires the use of additives that are emissive. Tertiary amines such as triethylenediamine and bis(dimethylaminoethyl)ether are common standards used by the industry to produce polyurethane foams. The speed of production and final physical properties are a reflection of the catalyst combination. Although these catalysts are widely used, there are several environmental and quality issues resulting from the volatility of these products.
One of the strategies used to reduce emissions from foam was based on introducing functional groups on tertiary amine catalysts able to react with isocyanate functionality. Using this approach, the tertiary amine catalysts would remain covalently bonded to the polyurethane polymer preventing its release into the environment. This approach can have some limitations because the functionalized tertiary amine can react with isocyanate prematurely causing undesired side effects such as polymer chain termination which would result in poor physical properties, excessive cell opening or foam collapse or excessive cross linking which can result in extensive shrinkage and poor dimensional stability. Another alternative approach to reduce odor and emissions is based on utilizing materials with increasing molecular weight and/or polarity. Products such as dimethylaminopropyl urea, bis(dimethylaminopropyl)urea, bis(dimethylaminopropyl) amine and N,N-bis(dimethylaminopropyl)-N-(2-hydroxypropyl) amine can provide acceptable physical properties as compared to industry standards whereas most conventional reactive catalysts cannot always achieve today's consumer and manufacturer requirements. Using these catalysts can reduce significantly the overall emissions from foam. However, the finished articles produced are by no means emissions-free and VOC and (FOG) values can reach several hundred ppm according to VDA 278 detection method.
When making flexible polyurethane foam, carboxylic acids are normally added to the polyurethane formulation to slow down the activity of the tertiary amine and prevent a fast increase in viscosity which allows for a more efficient mold-filling operation particularly in cases where molds with complex shapes and geometries are needed. This approach allows filling of small cavities and voids minimizing the number of defective articles. Acids most commonly used for this purpose are monoacids such as acetic acid, propionic acid, butanoic acid, hexanoic acid, 2-ethylhexanoic acid and the like. These acids, however, are known to be weakly bounded to the polymer thereby causing significant emissions even in the presence of tertiary amines containing isocyanate reactive functionalities. The typical strategy used to minimize emission from acids is similar to that already described for tertiary amine catalysts. In other words, using carboxylic acids containing functional groups able to react with isocyanate can render the carboxylic acid immobilized in the polyurethane polymer. However, this approach has similar limitation as the ones already described for non-emissive tertiary amines. The functionalized carboxylic acid can react with the isocyanate causing undesired effects such as polymer chain termination which would result in poor physical properties, excessive cell opening or foam collapse as well as excessive cross linking which can result in excessive shrinkage and poor dimensional stability depending on the choice of functionalized acid.
US Patent Publication No. US2007/0197760 A1 discloses a catalyst for production of polyurethane which is non-corrosive and exhibits effective delay of catalyst action. The catalysts comprises a mixture of a emissive tertiary amine and a saturated carboxylic acid represented by the general formula HOOC—(CH2)n—COOH where n is an integer form 2 to 14. However, the combination of catalyst and acid employed in US2007/0197760 A1 lead to foam with high emissions.
U.S. Pat. No. 6,387,972 disclosed an improved process to enhance polyurethane foam performance. Polyurethane foam is prepared by reaction between a polyisocyanate and a polyfunctional isocyanate-reactive component wherein said reactions are conducted in the presence of catalysts composed of (a) a specific reactive tertiary amine catalysts selected form the group consisting of bis(dimethylaminopropyl)amino-2-propanol, bis(dimethylaminopropyl)amine, dimethylaminopropyldipropanolamine, bis(dimethylamino)-2-propanol, N,N,N′-trimethyl-N′-hydroxyethyl-bis(aminoethyl)ether and mixtures thereof; and (b) at least one carboxylic acid salt of the specific reactive tertiary amine compound selected from the group consisting of hydroxyl-carboxylic acid salts and halo-carboxylic acid salts. The hydroxyl carboxylic acid results in lower emissions form foam due to the immobilization caused by its reaction with isocyanate. However, this approach has the disadvantage that excess isocyanate is required to render the acid immobilized and this reaction adversely influence the physical properties of polyurethane foam. Also, the immobilized acid can not freely migrate through the foam making it an ineffective blocking agent.
U.S. Pat. No. 6,432,864 disclosed novel acid-blocked amine catalyst and their use in the preparation of polyurethane foams. The acid-blocked amine catalyst is typically made by combining a tertiary amine with an acid prepared from an organic cyclic anhydride and a glycol. Since the acid produced by the reaction of a glycol with an anhydride yields a product containing a hydroxyl group, polyurethane foam made with this acid requires additional isocyanate to account for the OH group. In addition, the alcohol-acid can stop polymer growth due to the presence of a single hydroxyl group in the molecule which can have a detrimental impact on foam physical properties. Furthermore, the process yields an ester functionality that is hydrolytically unstable causing changing in reactivity as the polyurethane formulation system ages.
U.S. Pat. No. 4,066,580 disclosed a process for the manufacture of urethane groups containing polyisocyanurate foam produced by mixing polyisocyanates and polyols and frothing agents, auxiliaries and additives in the presence of a co-catalysts system which consists of a 1,3,5-tris(N,N-dialkylaminoalkyl)-s-hexahydrotriazine and an organic mono- or polycarboxylic acid, a polyesterol with an acid number greater than 4 or a mono-ester of polycarboxylic acids. In particular, the invention describes a method in which the cream time and rise times can be varied by using a particular catalysts consisting of 1,3,5-tris(N,N-dialkylaminoalkyl)-s-hexahydrotriazine and organic carboxylic acids, monoesters of the acids or polyesterols with acid numbers greater than 4. Thus the invention uses acids and polyacids but with the purpose of controlling the processing conditions of a PIR foam made specifically with 1,3,5-tris(N,N-dialkylaminoalkyl)-s-hexahydrotriazine.
U.S. Pat. No. 7,351,859 disclosed a tertiary amine-carboxylic acid salt, where the carboxylic acid and tertiary amine are selected such that the catalyst salt is blocked at room temperature and becomes unblocked at an elevated temperature. The compound is useful as a heat activated urethane catalyst. This invention disclosure relates specifically to the use of oxalic acid and/or salicylic acid in combination with a certain tertiary amine catalyst which may contain an isocyanate reactive group. This invention is not directed to make humid aged stable and low emission polyurethane foam but rather to a process to make foam in which tertiary amine carboxylate salts of oxalic acid and salicylic acid de-blocked at higher temperatures. The use of Polycat®8, or Polycat®5 would result in foam products with high emissions due to the lack of an isocyanate reactive group. On the other hand, Dabco®, Dabco®DMEA and Jeffcat®ZR70 will result in foam with poor ambient and humid aged physical properties as shown in example 1. Polycat®15 can give foam with acceptable physical properties and emissions. However, Polycat®15 used in combination with oxalic acid can have several environmental issues. Standard emission tests of flexible foam are normally carried out at 120° C. and oxalic acid sublimes at temperatures of 100° C. Therefore, any of these catalyst-oxalic acid combinations generates corrosive emissions which are unacceptable for use, for example, in automotive applications where end consumers could be exposed to acid and corrosive emanations. Oxalic acid is particularly highly corrosive due to it very high acidity (pKa=1.27). This also present a serious issue to the automotive interior because during the summer days temperatures inside a car can be very high causing oxalic acid emissions to corrode metal parts or damage other materials. On the other hand, salicylic acid has the disadvantage of having an isocyanate reactive group which has the limitations of chemical mobility and polymer chain termination discussed above.
U.S. Pat. No. 5,464,560 discloses an organic acid having at least two carboxylic groups, no tertiary amine groups and a pKa of up to 3.1 is added to an HCFC blowing agent or to a composition, preferably an isocyanate-reactive composition in which an HCFC blowing agent is present to reduce decomposition of the HCFC blowing agent. The resultant compositions are useful in the production of polyisocyanurate, polyurethane and/or polyurea foams. The use of organic acid in the presence of volatile HCFC will produce foam with high emission levels highly undesired in open cell flexible foam applications.
U.S. Pat. No. 6,576,682 relates to the combined use of metal salts of ricinoleic acid with reactive amine catalysts in the preparation of polyurethane foams and their solutions in aqueous or organic solvents in the preparation of polyurethane foams in combination with reactive amines. The purpose of the invention is to reduce emissions from volatile acids resulting from the hydrolytic decomposition of tin octoate salts. This patent describes emissions caused by organic carboxylic mono-acids such as 2-ethylhexanoic acid and how by increasing the molecular weight of the acid and incorporating hydroxyl functionality as in ricinoleic acid helped minimizing acid emissions. It also illustrates the fact that using standard organic carboxylic acids of formula R—CO2H in which R is hydrogen or alkyl group can lead to organic emissions from foam.
The disclosure of the previously identified patents and patent application is hereby incorporated by reference.
There is a need in this art for an improved foam and foam manufacturing method that has reduced emissions.